Electronic devices, such as cellular telephones, cameras, and computers, commonly use image sensors to capture images by sensing light. A typical imager sensor includes a focal plane array of pixels, and each pixel includes a photosensor, such as a photogate, photoconductor, or photodiode, for accumulating photo-generated charge in a portion of the substrate. When photons impinge on the photosensor, electron-hole pairs are generated. Conventional image sensors convert the electrons that are integrated (collected) in the pixels into a voltage, and the holes are generally discarded into the substrate.
After completion of integration cycle, collected electron charge is converted into a voltage, which is supplied to an output terminal of the image sensor. In a CMOS image sensor, the charge-to-voltage conversion is accomplished directly in the pixels themselves and the analog pixel voltage is transferred to the output terminal through various pixel addressing and scanning schemes. The analog signal can also be converted on-chip to a digital equivalent before reaching the chip output.
The pixels utilize a buffer amplifier, typically a source follower (SF) transistor to drive the sense lines that are connected to the pixels by suitable addressing transistors. After charge-to-voltage conversion is completed and the resulting signal is transferred out from the pixels, the pixels are reset in order to be ready for accumulation of new charge. In pixels that utilize a floating diffusion (FD) node as a charge detection node, the reset is accomplished by turning on a reset transistor that conductively connects the FD node to a voltage reference, which is typically the pixel SF drain node. This step removes collected charge, however, it generates kTC-reset noise. This kTC-reset noise is generally removed from the signal by a correlated double sampling (CDS) signal processing technique in order to achieve the desired low noise performance.
The typical CMOS image sensors that utilize the CDS concept usually require three (3T) or four transistors (4T) in the pixel—one of which serves as a charge transferring transistor. However, it is difficult adapting it for the high dynamic range (HDR) operation, where a large amount of charge must be stored in the pixels. Conventional methods for addressing this issue include: assigning to some sensor rows or pixels in a group of pixels a shorter integration time; using a logarithmic charge-to-voltage conversion characteristic; and incorporating charge storage capacitors into the pixels. These conventional methods, however, are not ideal because one or more may result in sacrificing the low light level resolution, a shorter integration time may lead to missing the detection of some short pulse duration light sources; logarithmic charge-to-voltage conversion may result in higher signal noise; and/or increasing the overall size of the chip due to having elements that occupy a large area in the pixel.